Advanced Search Problem

For advanced search (widget), I want to add a drop down list where users will have to select of cities.
Drop down list ‘city’, must also be included in the submission form where user will have to select their respective ‘city’ when submitting a particular listing.
Therefore the advanced search will have to search for a specific ‘city’ when selected, in relation with the ‘city’ in the submission form.
In General Setup>Search, I have already added a new field ‘city’.
Next:
Select a field to search
What field should I select?
For the submission form, I have also added ‘city’ in submission>custom field, and inserted all cities in the default values.
And how to get all default values of cities that I have inserted in submission>custom field?
I cant figure how to do it.
Please help, I badly need to complete it as soon as possible.
Thank you!
Directory Press 7.1.3